The complaints to the Conservative Party against Mr Johnson will be looked at by an independent panel. It could refer him to the party’s board, which has the power to expel him.

A source close to Mr Johnson offered no comment on the move while a Tory spokesman said: “The code of conduct process is strictly confidential.”

However, Mr Mitchell said Mr Johnson had “expressed himself in quite colourful language, but he hasn’t committed any offence and we need to be very careful about our handling of this because we believe in free speech in this country”.

“We must be careful not to go over the top on this. There is a procedure within the party if people complain then quite rightly the party has a mechanism for dealing with that. And that should take place,” he told Newsnight.

Constituency protest

The Conservative Party has been accused of not doing enough to tackle anti-Muslim prejudice in its ranks, despite an initiative to boost tolerance and diversity.

The party’s code of conduct states that Tory officials and elected representatives must “lead by example to encourage and foster respect and tolerance” and not “use their position to bully, abuse, victimise, harass or unlawfully discriminate against others”.

The independent panel will decide whether to refer Mr Johnson to the party’s board. Possible action includes suspension of membership or expulsion from the party among other, lesser sanctions.

Conservative Party chairman Brandon Lewis and leader Theresa May have both called on Mr Johnson to apologise for his comments.

A hundred Muslim women who wear the niqab or burka signed a letter to Mr Lewis, calling on him to withdraw the Conservative whip from Mr Johnson and launch an independent inquiry into Islamophobia in the party.

“We are not forced to make these clothing choices, nor are we oppressed,” the women write in their letter, which has been issued to the media by the Muslim Council of Britain.

Meanwhile more than 30 protesters gathered in Mr Johnson’s constituency of Uxbridge, calling for his resignation.

News of the Conservative Party investigation came as a watchdog which advises ex-ministers on taking new jobs ruled that Mr Johnson broke ministerial rules by taking up his weekly column at the Telegraph without consulting it first.